Enzyme regulation

What are the two major axes of enzyme regulation?

Activity of existing enzyme molecules (fast) and enzyme amount (slow).

Which mechanisms rapidly change enzyme activity?

Allosteric effectors, reversible phosphorylation, and proteolytic activation.

Which mechanisms change enzyme amount?

Transcriptional induction/repression and changes in synthesis or degradation.

Why is activity regulation faster than amount regulation?

Existing enzymes can be modified in seconds to minutes; changing enzyme abundance requires synthesis or degradation.

What is allosteric regulation?

Binding at a regulatory site changes enzyme conformation and activity.

What curve is typical of cooperative allosteric enzymes?

A sigmoidal substrate-velocity curve.

Why is the allosteric curve sigmoidal?

Binding at one site influences other subunits, producing cooperativity.

What does an allosteric activator generally do to the curve?

Shifts it left, increasing activity at a given substrate concentration.

What does an allosteric inhibitor generally do to the curve?

Shifts it right, decreasing activity at a given substrate concentration.

Why is cooperativity useful in metabolic control?

It creates a switch-like response over a narrow substrate-concentration range.

How does allosteric inhibition differ from competitive inhibition?

Allosteric inhibition uses a regulatory site; competitive inhibition directly competes with substrate at the active site.

What is feedback inhibition?

An end product inhibits an upstream enzyme in its own biosynthetic pathway.

Why does feedback inhibition usually target the first committed step?

It prevents wasteful use of precursors and prevents downstream intermediates from accumulating.

What is the metabolic 'thermostat' idea behind feedback inhibition?

More end product decreases its own further production.

What enzyme does isoleucine inhibit in the lecture example?

Threonine deaminase.

What enzyme does CTP inhibit in the lecture example?

Aspartate transcarbamoylase.

What is reversible covalent enzyme regulation?

A chemical group is added or removed, commonly phosphate by kinases and phosphatases.

Which amino acids are commonly phosphorylated?

Serine, threonine, and tyrosine.

What does phosphorylation add chemically?

A bulky, negatively charged phosphate group that can alter conformation or binding.

Can phosphorylation activate one enzyme and inhibit another?

Yes; the effect depends on the specific enzyme.

What is a zymogen?

An inactive enzyme precursor activated by specific proteolytic cleavage.

Why is zymogen activation effectively irreversible?

A peptide bond is cleaved, and removing a phosphate cannot reverse that structural change.

Why are digestive proteases synthesized as zymogens?

To reduce the risk of autodigestion before they reach the correct site.

What is the classic digestive zymogen example?

Trypsinogen is cleaved to trypsin.

Where should trypsinogen normally be activated?

In the duodenum.

Why is premature trypsinogen activation dangerous?

Active trypsin can trigger pancreatic autodigestion.

Where else does zymogen activation play a major role?

Blood coagulation, complement activation, and caspase activation in apoptosis.

What is the key contrast between phosphorylation and proteolysis?

Phosphorylation is reversible; proteolytic activation is a one-way switch.

What is enzyme induction?

Increased transcription and synthesis of an enzyme.

What is enzyme repression?

Reduced transcription and synthesis of an enzyme.

How can degradation regulate enzyme abundance?

Targeted proteolysis removes enzyme molecules and shortens their functional lifetime.

What determines steady-state enzyme abundance?

The balance between synthesis and degradation.

Why is enzyme induction relatively slow?

New protein must be synthesized.

Why can enzyme induction produce sustained changes?

It increases the number of enzyme molecules available for catalysis.

Why is cytochrome P450 induction an example of the amount axis?

Induction increases the amount of CYP enzyme rather than simply changing the activity of existing molecules.

What is an isozyme?

A molecular form of an enzyme that catalyzes the same or related reaction but can differ in tissue distribution or properties.

Why are isozymes useful diagnostically?

Their tissue distributions can help identify the source of tissue injury.

Which diagnostic proteins are highlighted with enzyme amount?

Creatine kinase, lactate dehydrogenase, and troponin.

What is cytochrome P450?

A superfamily of heme-containing monooxygenases involved in Phase I metabolism.

Where are many CYP450 enzymes located?

In hepatic smooth endoplasmic reticulum.

What does CYP450 primarily do in Phase I metabolism?

Oxidizes substrates and adds or exposes functional groups.

What other reaction types are included in the lecture's Phase I description?

Oxidation, reduction, and hydrolysis.

Why is CYP450 called a heme enzyme?

Its active site contains a heme group involved in oxygen activation.

Why is it called P450?

The heme-containing enzyme gives a characteristic absorbance near 450 nm when bound to carbon monoxide.

What are the key reactants needed for CYP450 oxidation?

Molecular oxygen and NADPH.

What happens to the two oxygen atoms during CYP450 monooxygenation?

One oxygen is incorporated into the substrate and the other is reduced to water.

What is the overall purpose of Phase I metabolism?

To add or expose a functional group that can facilitate further metabolism and excretion.

Does Phase I metabolism always inactivate a drug?

No. It can inactivate, activate, or create a toxic metabolite.

Which CYP450 isoform metabolizes many clinical drugs?

CYP3A4.

Which major CYP450 isoforms are emphasized?

CYP3A4, CYP2D6, CYP2C9, CYP1A2, and CYP2E1.

Why is CYP2D6 clinically important?

Genetic variation can substantially alter metabolism and drug response.

Why is CYP2C9 clinically important?

Genetic variation affects metabolism of drugs such as warfarin.

What factors can alter CYP450 activity?

Genetics, other drugs, diet, and disease.

What is CYP induction at the molecular level?

An increase in production of a CYP enzyme, raising the amount available to metabolize substrates.

What is the expected effect of a CYP inducer on an active substrate drug?

Faster metabolism, faster clearance, lower drug levels, and possible therapeutic failure.

Why does CYP induction take days?

New enzyme protein must be synthesized.

Which CYP inducers are listed in the lecture?

Rifampin, phenytoin, carbamazepine, phenobarbital, St. John's wort, smoking for CYP1A2, and chronic ethanol.

What is the expected effect of a CYP inhibitor on an active substrate drug?

Slower metabolism, reduced clearance, higher drug levels, and possible toxicity.

Why are CYP inhibition effects often faster than induction effects?

Inhibition can directly block existing enzyme without requiring new protein synthesis.

Which CYP inhibitors are listed?

Azole antifungals, macrolides, grapefruit juice affecting intestinal CYP3A4, ritonavir, cimetidine, isoniazid, and acute ethanol.

What is the high-yield difference between CYP induction and inhibition?

Induction generally lowers substrate drug levels; inhibition generally raises them.

Why can CYP induction cause therapeutic failure?

The drug is cleared faster and may fall below its effective concentration.

Why can CYP inhibition cause toxicity?

Reduced clearance allows the active drug to accumulate.

What CYP2D6 phenotypes are listed?

Poor, intermediate, extensive, and ultra-rapid metabolizers.

Why is codeine a classic pharmacogenomic example?

CYP2D6 converts codeine to morphine, so genetic differences alter active metabolite formation.

What can happen in a CYP2D6 poor metabolizer taking codeine?

Little morphine is formed, so analgesic effect can be reduced.

What can happen in a CYP2D6 ultra-rapid metabolizer taking codeine?

More rapid morphine formation can increase toxicity risk.

Which genetic factors affect warfarin response in the lecture?

CYP2C9 and VKORC1 variants.

What is pharmacogenomics?

Using genetic variation to predict differences in drug metabolism or response and guide treatment.

What is a prodrug?

A drug that requires metabolic activation to become pharmacologically active.

What happens when a CYP inhibitor is given with a prodrug?

Activation can fall, reducing efficacy.

What happens when a CYP inhibitor is given with an active drug normally inactivated by CYP?

Clearance falls, drug levels rise, and toxicity risk can increase.

Which prodrugs are named in the lecture?

Codeine and clopidogrel.

What question should you ask first when predicting a CYP drug interaction?

Is the CYP enzyme activating the drug or inactivating the drug?

Why can the same CYP inhibitor produce opposite clinical outcomes with different drugs?

One drug may require CYP activation while another is cleared by CYP-mediated inactivation.

Why can grapefruit juice raise concentrations of some oral drugs?

It can inhibit intestinal CYP3A4 and decrease first-pass metabolism.

Why can smoking alter drug metabolism?

Smoking can induce CYP1A2.

What is the chronic-versus-acute ethanol distinction for CYP2E1?

Chronic ethanol induces CYP2E1 by increasing enzyme amount; acute ethanol can inhibit CYP-mediated metabolism.

Why is chronic versus acute ethanol a high-yield distinction?

The same exposure can produce opposite metabolic effects depending on duration.

What is the best mental model for enzyme regulation?

First ask whether the cell is changing what existing enzymes do or changing how many enzyme molecules exist.

Which regulation strategy is best for an immediate metabolic response?

Allostery or reversible phosphorylation.

Which regulation strategy is best for a sustained change in metabolic capacity?

Induction, repression, or altered degradation.

Why are zymogen cascades useful in coagulation and complement?

Sequential proteolysis allows rapid, amplifying, irreversible activation.

What happens when an end product inhibits the first committed enzyme and shifts a sigmoidal curve right?

It is allosteric feedback inhibition.

A regulatory molecule binds away from the active site and shifts an enzyme's sigmoid left. What does this suggest?

Allosteric activation.

An inactive precursor becomes active after one peptide bond is cleaved. What mechanism is involved?

Zymogen activation.

An enzyme's activity changes within minutes after addition of a phosphate group. What regulation is involved?

Reversible covalent phosphorylation.

A medication increases transcription of a CYP enzyme. Which regulatory axis is involved?

Enzyme amount.

A drug starts being cleared faster several days after a second medication is added. What mechanism is most likely?

CYP induction.

A drug level rises rapidly after addition of another drug that directly blocks its metabolism. What mechanism is most likely?

CYP inhibition.

A patient has genetically low CYP2D6 activity. What concept explains the altered response?

Pharmacogenomic variation and poor-metabolizer phenotype.

Why can enzyme degradation change metabolic flux without changing gene transcription?

Removing enzyme decreases the number of active catalytic molecules available.

Why does changing enzyme amount generally affect maximum pathway capacity?

More enzyme molecules provide more total catalytic capacity; less enzyme lowers pathway capacity.

What is the difference between feedback inhibition and enzyme repression?

Feedback inhibition changes activity of an existing upstream enzyme; repression decreases future enzyme synthesis.

What is the difference between allostery and phosphorylation?

Allostery uses binding of an effector at a regulatory site; phosphorylation covalently adds a phosphate group.

What is the difference between phosphorylation and proteolytic activation?

Phosphorylation can be reversed by phosphatases; proteolytic cleavage is effectively permanent.

What is the difference between CYP inhibition and general allosteric inhibition?

CYP inhibition is a drug-enzyme interaction affecting metabolism, whereas allostery is a broader cellular regulatory strategy involving regulatory-site binding.

Why does CYP inhibition of an active drug often increase adverse effects?

The drug is not cleared as efficiently, increasing exposure.

Why does CYP induction of an active drug often decrease therapeutic effect?

The drug is cleared faster, reducing exposure.

Why does CYP induction of a prodrug not necessarily decrease its effect?

Greater metabolism can increase formation of the active metabolite if CYP is the activation step.